abstract As history shows, some human beings are capable of acting very immorally. 1 Technological advance and consequent exponential growth in cognitive power means that even rare evil individuals can act with catastrophic effect. The advance of science makes biological, nuclear and other weapons of mass destruction easier and easier to fabricate and, thus, increases the probability that they will come into the hands of small terrorist groups and deranged individuals. Cognitive enhancement by means of drugs, implants and biological (including genetic) interventions could thus accelerate the advance of science, or its application, and so increase the risk of the development or misuse of weapons of mass destruction. We argue that this is a reason which speaks against the desirability of cognitive enhancement, and the consequent speedier growth of knowledge, if it is not accompanied by an extensive moral enhancement of humankind. We review the possibilities for moral enhancement by biomedical and genetic means and conclude that, though it should be possible in principle, it is in practice probably distant. There is thus a reason not to support cognitive enhancement in the foreseeable future. However, we grant that there are also reasons in its favour, but we do not attempt to settle the balance between these reasons for and against. Rather, we conclude that if research into cognitive enhancement continues, as it is likely to, it must be accompanied by research into moral enhancement.